A three-dimensional finite element model has been developed to study the effects of solder volume and pad size on the reliability of a 540-I/O plastic ball grid array (PBGA) assembly. The model consisted of four parts: solder joint profile model, global model for the PBGA assembly, local model for the single solder joint, and solder joint fatigue life calculation. A submodeling technique was used to transfer the displacements to the local model for a detailed analysis under thermal cycling conditions. By using this model, the effects of solder volume and pad size on the fatigue life of BGA solder joints have been studied. With different volumes or pad sizes, the fatigue life could change from 1800 to 15000 thermal cycles. Since the fatigue life of this PBGA assembly was dominated by local mismatch in most cases, short joints could have longer fatigue lives than those with long joints. In addition, the effect of pad size on fatigue was also increased from second order to a higher order relationship. Such uncommon conclusions resulted from strong coupling effects of solder joint shape, global mismatch, and local mismatch